Problem: The scale on a map is 3cm : 10km. If two cities are 21cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 3cm on the map corresponds to an actual distance of 10km. A distance of 21cm on the map is the same as 7 $\cdot$ 3cm. The actual distance between the two cities is 7 $\cdot$ 10km, or 70km.